Heating element for vacuum furnace



1968 H. E. MESCHER ETAL 3,368,022

HEATING ELEMENT FOR VACUUM FURNACE I Filed June 10, 1966 INVENTORS HAROLD E. MESCHER WALTER E. HEYER BY wag/M3;

ATTORNEY IVE HEATING ELEMENT FQR VA'CUUM FURNACE Harold E. Mescher, Pico Rivera, and Walter E. Heyer,

Los Alamitos, Califi, assignors to Pacific Scientific Company, City of Commerce, Calif., a corporation of California Filed June 10, 1966, Ser. No. 556,643 Claims. (Cl. 13--3l) This invention relates generally to electrical heating ele ments for cold-wall vacuum furnaces of the type suitable for metallurgical uses such as sintering, heat-treating, and brazmg operations, and the invention has reference more particularly to a novel tubular metallic heating element.

Heating elements as used heretofore in cold-wall vacuum furnaces as of the horizontal design type are generally of two kinds, i.e., either the rod or strip-overbend type supported on refractory spools attached to an internal frame of the furnace radiation shields, or of the tubular graphite type that extends through clearance holes in the shield pack and is supported at either end by graphite terminal pieces, this latter type being the commonly used heating element. Heating elements as heretofore constructed are not satisfactory in use because, in the case of the rod or strip-overbend type, these are not only expensive since a lot of expensive metal is used in forming the heating elements, but they tend to distort and sag m use and have a somewhat short life. The tubular graphite type commonly used is objectionable in that when treating many materials in the furnace the carbon vaporizes from the graphite elements and will discolor or alter adversely the properties of the material being treated.

The principal object of the present invention is to pro vlde a novel extremely thin-walled tubular heating element that can be easily fabricated at a relatively low cost because of the limited amount of expensive metal used in producing the electrode, the said heating element having novel terminal means designed to provide good electrical contact with the heating element and its terminal means while at the same time preventing overheating of the connected leads in use.

A feature of the present invention is to provide a novel thin-walled tubular heating element that is formed simply by folding a sheet of metal that can withstand high temperatures, such as molybdenum, into a cylinder with the longitudinal seam left open, thereby eliminating welding and its disadvantages, said heating element being mounted at its ends in sockets provided in its terminal means and retained therein by the wedging action of a tapered bolt, thus enabling the ready connection and disconnection of the tubular heating element with respect to its terminal means while providing for extremely good electrical connection between these members, the terminal means of the heating element being made preferably of an inexpensive alloy that need not withstand high temperatures.

Another feature of the invention is to provide a novel heating element that is non-gaseous and can be used when treating any type of material without injury to the latter, the said heating element being relatively lightweight compared to prior art heating elements and being easily installed and replaced when desired.

These and other features and advantages of the present invention will be more apparent after a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a sectional view, with parts broken away, showing the novel heating element as used in a typical vacuum furnace; and

FIG. 2 is an enlarged sectional view taken along line 22 of FIG. 1.

Similar characters of reference are used in the above figures to designate corresponding parts.

3,368,622 Patented F el). 6, 1968 Referring now to the drawings, the reference numeral 1 designates a cold-wall vacuum furnace, as of the horizontal type, suitable for metallurgical uses such as sintering, heat-treating and brazing operations. The cold-wall furnace 1 which may be of a type somewhat similar to that disclosed in US. Patent #3,185,460 has a wall panel 16 with openings therein into which project ceramic sleeves 2, said sleeves being secured therein as by cementing. Within the sleeves Z are positioned cylindrical terminal members 3 which are slidable into the hollow interior of the sleeves 2. The terminal members 3 can be of any metal or alloy which can withstand reasonable temperatures, such as of Inconel, since these members are protected from the intense heat of the furnace by the ceramic sleeves 2 to a certain extent. Each of the terminal members 3 is bored longitudinally, as indicated at 4, to receive a bolt 5 which is formed with a tapered head 6 of frusto-conical shape. The terminal member 3 is provided with an enlarged cylindrical recess or socket 9 at its inner end forming a shoulder 10 against which the tubular heating element of this invention is adapted to abut, as will further appear. The bolt head 6 is adapted to conformably engage substantially semi-circular matching sleeves 7 and 7 whose interior surfaces are frusto-conical for conformably engaging the bolt head 6. The outer surfaces of sleeves 7 and 7 ar cylindrical for conforming to the inner surface of a thin-walled tubular heating member 8. The two semi-circular sleeves 7 and 7 in effect form a split sleeve, cylindrical on the outside and tapered on the inside, and may be of the same material as the terminal member 3, such as Inconel.

The thin-walled tubular heating member 8 may be formed by taking a rectangular sheet of thin metal capable of withstanding high temperatures, such as molybdenum or tungsten alloy, having a thickness of the order of .010", and by rolling the same on an arbor to the desired tubular shape with the two edges of the sheet abutting each other to form an open seam. To assemble the tubular heating member 8 in its terminal member 3, the split tapered sleeves 7, 7' are placed in the cylindrical recess 9 of the terminal member 3 so that the same will abut the shoulder 10. The bolt 5 is passed through the interior of the tubular heating member 8 with its threaded end foremost to the point where the matching tapers of the head 6 and sleeves 7 and 7 start to engage, whereupon the end of the heating element 8 is placed in the recess 9 between the tapered sleeves 7, 7 and the cylindrical wall of the terminal body recess 9. The cylindrical outer surfaces of the tapered sleeves 7, 7 will press against the inner wall of the heating member 8 and can be forced tightly thereagainst by drawing up on the nut 11 threaded on the outer portion of the bolt 5. This enables the tubular heating member 8 to be forced into tight electrical contact with the terminal member 3, the open seam 17 of the member 8 permltting ready expansion of this member.

Preferably, the terminal body 3 is provided wlth a copper washer 12 on its outer end which may be secured thereto as by silver soldering in order to make good electrical contact between a connector-strip terminal 13 and the terminal body 3, the nut 11 engaging the exterior of terminal 13 and forcing it into firm electrical contact with the washer 12. A copper radiation fin or disc 14 is preferably inserted between the terminal connector 13 and the copper washer 12, which radiation shield serves to provide protection for the terminal connector 13 against the heat of the furnace. It will be noted that in use the copper radiation shield 14 will radiate heat to the water-cooled furnace panel 16 of the furnace and to the water-cooled shell of the furnace, thereby protecting the connector 13 against overheating. By tightening nut 11, the tapered head 6 of bolt 5 will press sleeves 7 and 7' outwardly and force the end portion of the tubular heating member 8 into snug electrical contact with the terminal member 3 so that excellent electrical connection is made between terminal connector 13 and the heating member 8. The tapered sleeve 7', 7 is kept in alignment with the bolt by the presence of the shoulder 10 abutting against the sleeve members 7 and "I" under the pressure of the tapered head 6. The bolt 5 is provided with flats so that the bolt can be held stationary while the nut 11 is being loosened or tightened. By merely loosening this nut, it is possible to quickly disconnect the tubular heating member 8 from its terminal member 3, as where it is desired to replace the heating member 8 after the same is worn out. The ceramic sleeve 2 provides electrical insulation between the furnace proper and the heating element.

Thus, it will be seen that the novel heating element of the present invention not only is economical to manufacture since tubular member 8 is formed from a thin flat sheet of expensive metal rather than a solid body, but has a long life in use since the heavier terminals comprising members 3, bolts 5, and fins 14 are good heat conductors serving to carry heat away. from the ends of the heating member. These parts are of less expensive metal than that of tube 8 and are readily machined. No welding is required in producing the novel heating element, and this not only eliminates such step, but prevents distortion of the heating element due to welding. The silver soldering of the Washer 12 to the end of the terminal member 3 is a relatively simple operation and can be done by hand.

The all-metallic nature of the novel heating element and its terminals permits its use in furnaces where graphite cannot be tolerated, and it will be noted that a minimum amount of ceramic is used to get the desired metallurgical results. The novel heating member 8 of this invention is also relatively light, since it is hollow, and has an extremely large heat radiating surface relative to the amount of metal used, and this is highly desirable in use for a solid member tends to overheat interiorly, and further, heat does not have to pass outwardly from deep within the metal to the external surfaces thereof, but allthe heat is generated substantially at the external surface of the member, and hence is radiated quickly into the furnace without overheating the member itself, thereby providing for the long life of the same. The easily replaceable nature of the member 8 in use also is a highly desirable feature as it cuts the down-time of the furnace.

Since many changes could be made in the above construction of the novel heating element of this invention and many apparently widely different embodiments of this invention could be made Without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A heating element for vacuum furnaces comprising a thin-walled tubular member of high-temperattire-resistant metal, said tubular member being rolled from a fiat sheet and having an open longitudinal seam, a terminal member having a cylindrical recess receiving an end portion of said tubular heating member, and wedging means within said recess forcing said tubular member outwardly into intimate electrical contact with the wall of said recess and hence with said terminal member, the tubular member seam permitting such expansion of said member.

2. A heating element as defined in claim 1 wherein said terminal member is longitudinally apertured contiguous to said cylindrical recess, said wedging means comprising a bolt having a frusto-conical head and tapered sleeve means interposed between said bolt head and said tubular member.

3. A heating element for vacuum furnaces as defined in claim 2 wherein said bolt is provided with a nut for drawing against said terminal member to thereby force said sleeve means into intimate contact with the inner surfaces of said tubular heating member, thereby forcing the latter into tight electrical contact with said terminal member.

4. A heating element as defined in claim 3 comprising an insulating bushing extending into the vacuum furnace, said bushing serving to support said terminal member therewithin, a strip terminal, and a radiating fin, said terminal and fin being interposed between said nut and said terminal member, thereby providing a cool strip terminal connection for said heating element.

5. A heating element as defined in claim 4 wherein said sleeve means comprises a pair of semi-circular sleeve members having cylindrical outer surfaces for contacting said tubular sleeve member and frusto-conical inner surfaces for contacting said bolt head.

References Cited UNITED STATES PATENTS 3,057,936 10/1962 Hill 13-25 ROBERT K. SCHAEFER, Primary Examiner.

M. GINSBURG, Assistant Examiner. 

1. A HEATING ELEMENT FOR VACUUM FURNACES COMPRISING A THIN-WALLED TUBULAR MEMBER OF HIGH-TEMPERATURE-RESISTANT METAL, SAID TUBULAR MEMBER BEING ROLLED FROM A FLAT SHEET AND HAVING AN OPEN LONGITUDINAL SEAM, A TERMINAL MEMBER HAVING A CYLINDRICAL RECESS RECEIVING AN END PORTION OF SAID TUBULAR HEATING MEMBER, AND WEDGING MEANS 